Case, I can't answer if the paint is original or not. It was often used as a coating for rough finished fiberglass.
The stuff you would want to get is called Zolatone. It was used in boats, car trunks, campers, and fiberglass horse trailers.
Very durable. Pricey, but tougher than the rattle can stuff they sell now. Auto body supply companies have it. Expect to pay $150 a gallon, or $50 a quart.